Thymidine can protect or rescue normal tissues from the cytotoxic effect of methotrexate while maintaining antitumor activity. This research is directed at further increasing the therapeutic ratio of methotrexate in combination with thymidine. Experiments will seek to identify human tumors intrinsically sensitive to methotrexate with thymidine by virtue of deficient purine and/or thymidine salvage pathways, or collaterally sensitive due to resistance to thiopurines. Included are studies of the major factors that control therapeutic ratio of methotrexate with thymidine, and the conditions that yield optimal selectivity against human tumors. Other studies will evaluate drugs that inhibit nucleoside membrane transport for their ability to augment the selectivity of thymidine protection or rescue. In addition, we will investigate the role of high local tumor concentrations of salvage pathway metabolites as a mechanism of resistance to methotrexate in vivo.